We have continued our studies on the regulation of the expression of human and rodent ribosomal RNA in rodent-human hybrids and found that the species specific block occurs at the transcription level. We have also found that we can reactivate the silent rRNA genes by infecting the hybrids with either polyoma or SV40 viruses. We are presently investigating the mechanism of suppression. We are also continuing our studies on the genetics of human immunoglobulins.